car wont idle when warm in gear

+1 on checking for vacuum leaks. I had a manifold leak on me that progressively got worse until I had the same issue as you. Replaced the manifold to head gasket, no problem. Spray some carb cleaner around the probable areas (manifold gaskets etc) but DON'T spray on the carb, as carburetor cleaner is powerful and will dissolve the components of the vacuum choke diaphragm.

Have someone hold the throttle so the car is running like your saying and staying alive when warm. Spray short, quick bursts around the manifold gaskets and carb base gasket. If the idle speed picks up and smooths out, then the vacuum leak will suck the carb cleaner into the leak thus richening the mixture. Once the leak is located, you know what to do.